1. Field of the Invention
The present invention generally relates to the development of an electrostatic latent image on a latent image carrier in electrophotographic systems. More specifically, the invention relates to a cloud development method whereby a toner cloud is electrically produced.
2. Description of the Related Art
A developing unit used in a conventional image forming apparatus, such as a copy machine, a printer, or a FAX machine, may employ a two-component developing method or a one-component developing method. The two-component developing method is suitable for high-speed development, and is employed in the majority of the current middle- to high-speed image forming apparatuses.
In the two-component developing method, in order to achieve a high image quality, it is necessary to make the developer, which typically consists of a toner formulation and magnetic material called a carrier, very fine and dense at the point where the developer comes into contact with an electrostatic latent image on the latent image carrier. For this purpose, the carrier particles dots of high-resolution. In this method, a wire to which a high-frequency bias is applied is disposed at the development portion, whereby a toner cloud is produced at the development portion with which to realize high-resolution dot development characteristics.
Japanese Laid-Open Patent Application No. 3-21967 discloses a method of forming an electric field curtain over a rotating roller in order to form a toner cloud stably and efficiently.
Japanese Laid-Open Patent Application No. 2003-15419 discloses a developing apparatus in which the developer is transported by an electric field curtain based on a traveling-wave electric field.
Japanese Laid-Open Patent Application No. 9-269661 discloses a developing apparatus having plural magnetic poles that cause a substantially single carrier layer to be substantially uniformly adsorbed on the peripheral surface of the developing roller.
Japanese Laid-Open Patent Application No. 2003-84560 discloses a developing apparatus in which electrodes are disposed on the surface of a developer carrier at regular intervals interposed with insulating portions. Predetermined bias potentials are applied to the electrodes in order to produce an electric field gradient near the developer carrier surface, causing a nonmagnetic toner to become attached to the developer carrier for transportation.
In connection with the two-component developing method, there is an increasing demand for higher image quality. To meet such a demand, the pixel dot size needs to be equal to or smaller than the current carrier particle sizes. Thus, it is necessary to make the carrier particles smaller from the viewpoint of individual dot reproducibility.
However, as the carrier size is reduced, the permeability of the carrier particle decreases, which causes the carrier to be more readily separated from the developing roller. If a separated carrier particle attaches to the latent image carrier, not only the image is made deficient by the attaching of the carrier particle per se, but also various other side effects may be caused, including the potential of the attached particle to damage the latent image carrier.
In order to prevent such carrier separation, various attempts have been made. One is the attempt to increase the permeability of the carrier particle from a material aspect. Another attempt has been to increase the magnetic force of the magnets contained in the developing roller. However, development of a suitable material for the magnets with increased magnetic force has been very difficult due to the need to balance cost reduction and image quality.
Furthermore, the size of the developing roller is becoming increasingly smaller due to the continuing demand for ever smaller sizes of equipment, making it more difficult to design a developing roller with a strong magnetic field configuration such that carrier separation can be completely prevented.
Because the two-component developing method inherently involves a process of forming a toner image by rubbing bristles of a two-component developer, called a magnetic brush, against an electrostatic latent image, the development characteristics of individual dots tend to become uneven due to the unevenness in the bristles.
While improved image quality may be obtained by forming an alternating electric field between the developing roller and the latent image carrier, the fundamental image unevenness due to the irregularities inherent in the developer cannot be completely eliminated.
In the one-component developing method, during the process of reducing the thickness of the toner layer on the developing roller by the toner regulating member, the toner is pressed against the developing roller rather strongly. As a result, the toner responds to an electric field at the development portion very poorly. Thus, normally, in order to obtain a high image quality, a strong alternating electric field is formed between the developing roller and the latent image carrier. However, even with such an alternating electric field, it is still difficult to develop the electrostatic latent image stably with a constant supply of toner. Thus, it has been difficult to develop high-resolution fine dots uniformly.
Another disadvantage of the one-component developing method is that the toner is subject to much stress during the formation of the toner thin-layer on the developing roller. As a result, the toner, which is circulated in the developing unit, degrades fast. As the toner degrades, unevenness tends to occur also in the step of forming the toner thin-layer on the developing roller. Thus, the one-component developing method is not generally suitable for forming an image at high speed and with high durability.
Some of the aforementioned problems may be overcome by a hybrid method, although with an increase in the size of the developing unit or the number of its components. However, there still remains the same problem at the development portion as in the one-component developing method. Namely, it is still difficult to develop fine and uniform dots with high resolution.
While the aforementioned method of Japanese Laid-Open Patent Application No. 3-113474 may be capable of realizing a stable and high-quality image development, the structure of the developing unit used is complicated.
The aforementioned method of Japanese Laid-Open Patent Application No. 3-21967 may be capable of reducing the size of equipment and achieving high-quality development. However, a study conducted by the present inventors showed that with this method, various conditions relating to the electric field curtain and development need to be strictly limited in order to obtain an ideal image quality. If an image is formed under inappropriate conditions, no intended effects are obtained and indeed a poorer quality image may result.
In an image forming process in which a first toner image is formed on the latent image carrier and then a second, and a third toner images are formed sequentially thereon, the toner image that is previously formed must not be disturbed. The contactless one-component developing method or the aforementioned toner cloud development method according to Japanese Laid-Open Patent Application No. 3-113474 are capable of forming a toner image of an individual color on the latent image carrier sequentially. However, in all of these methods, because an alternating electric field is formed between the latent image carrier and the developing roller, part of the toner may be peeled from the toner image previously formed on the latent image carrier and enter into the developing unit. As a result, not only the image on the latent image carrier is disturbed but also the toners of various colors may become mixed in the developing unit. This may be fatal for achieving a high-quality image. In order to overcome this problem, a development method is required that does not involve the formation of an alternating electric field between the latent image carrier and the developing roller.
While such a method may be realized with the aforementioned cloud development method of Japanese Laid-Open Patent Application No. 3-21967, this method requires strictly limited conditions to achieve intended effects, as mentioned above.
Japanese Laid-Open Patent Application No. 2002-341656 teaches a method whereby a toner is electrostatically transported by alternating electric fields of three or more phases while eliminating any mechanical driving of the toner carrier. In this method, however, the toner may accumulate on the transport substrate starting with the toner whose electrostatic transport has been terminated for one reason or another. While a structure to overcome this problem has been proposed by Japanese Laid-Open Patent Application No. 2004-286837, for example, which is based on a combination of a fixed transport substrate and a toner carrier that moves on the surface thereof, the mechanism involved is very complex.
In order to solve this problem, the present inventors have proposed a method whereby an electric field that changes periodically with time is produced between electrodes of two phases, causing the toner to move or hop away from the rotating toner carrier while the toner is carried to an area opposite the latent image carrier where the latent image is developed.
In this method, instead of the conventional one-component developing roller, electrodes of two phases are embedded in a roller (to be hereafter referred to as a flare roller) at fine pitches, and the toner is caused to move or hop over the roller surface. The electrodes are covered with an insulating surface protection layer.
A study conducted by the present inventors, however, showed that when the flare roller is rotated, the surface potential of the flare roller greatly varies for various reasons, such as the triboelectric charging between the toner layer thickness regulating member and the roller, the triboelectric charging between the hopping toner and the roller, and the injection of charge into the flare roller surface by the potential difference between an average bias applied to the supply roller and an average bias applied to the flare roller.
Consequently, at the portion opposite the latent image carrier, the potential difference between the flare roller surface and an image portion or a non-image portion of the latent image carrier may fluctuate, resulting in image density irregularities and/or scumming.